(a) Technical Field
The present invention relates to an electro-hydraulic power steering apparatus for an environment-friendly vehicle and a method of controlling the same. More particularly, it relates to an electro-hydraulic power steering apparatus for an environment-friendly vehicle which can prevent a hydraulic pressure for maintaining a steering angle of a steering wheel from being released when the steering angle of the steering wheel is maintained at not less than a particular angle, and a method of controlling the same.
(b) Background Art
In general, environment-friendly vehicles have been studied and developed as an alternative for decreasing fuel costs and reducing exhaust gas through improvement of fuel efficiency. In particular, in a case of commercial vehicles (e.g., buses, trucks, and the like) which utilized a large amount of oil, it is known that fuel efficiency thereof is remarkably improved.
Electro-hydraulic power steering (EHPS) apparatuses instead of engine mount hydraulic power pumps are typically mounted within the environment-friendly commercial vehicles (diesel hybrid buses or the like) or commercial vehicles, in order to improve fuel efficiency thereof.
For reference, an electro-hydraulic power steering (EHPS) apparatus refers to a steering apparatus for assisting in steering by actuating a hydraulic pump through an operation of an independent electric motor.
As shown in FIG. 1, an electro-hydraulic power steering apparatus mounted to an environment-friendly vehicle according to the related art includes a gear box 10 for assisting a steering force of a steering wheel, a reservoir tank 20 for storing hydraulic oil, and an electro-hydraulic power pump 30 for pumping the hydraulic oil by an electric motor to supply the hydraulic oil in the reservoir tank 20 to the gear box 10.
Referring to FIG. 2, the gear box 10 includes a worm shaft 11 connected to the steering wheel, a piston 12 elevated in internal contact with the worm shaft 11 when the worm shaft 11 is rotated in place, a sector gear 13 for transferring a force to a pitman arm and a drag link while being engaged with a worm formed on a surface of the piston 12 to be rotated, to perform steering of a vehicle, and an upper hydraulic chamber 14 and a lower hydraulic chamber 15 formed at an upper side and a lower side of the piston 12, respectively.
Further, a hydraulic oil supply line 16 that supplies the hydraulic oil to the upper hydraulic chamber 14 or the lower hydraulic chamber 15 is connected to an input end of the gear box 10, and a hydraulic oil return line 17 is connected to an output end of the gear box 10. Also, the electro-hydraulic power pump 30 is connected to an end of the hydraulic oil supply line 16, and the reservoir tank 20 is connected to an end of the hydraulic oil return line 17.
Here, an operation flow of the electro-hydraulic power steering apparatus according to the related art will be briefly described below.
First, when a signal of a steering angle detection sensor (not shown), which has detected a change in a steering angle of the steering wheel, is input to a controller, and an operation signal is transferred from the controller to the electro-hydraulic power pump 30 at the same time, and an electric motor (not shown) installed in the electro-hydraulic power pump 30 is operated.
Next, the hydraulic oil in the reservoir tank 20 flows along the hydraulic oil supply line 16 through the pumping operation of the electro-hydraulic power pump 30, and is introduced into the upper hydraulic chamber 14 or the lower hydraulic chamber 15 of the gear box 10.
When the worm shaft 11 is rotated in place and the piston 12 is raised by a steering operation of the steering wheel toward one side, the hydraulic oil introduced into the lower hydraulic chamber 15 of the gear box 10 provides pressure which upwardly pushes the piston 12 to assist a steering force of the steering wheel. At this time, the hydraulic oil in the upper hydraulic chamber 14 is discharged through a discharge port (not shown) in the upper hydraulic chamber 14, and returns to the reservoir tank 20 along the hydraulic oil return line 17 at the same time.
In contrast, when the worm shaft 11 is rotated in place and the piston 12 is lowered by a steering operation of the steering wheel toward an opposite side, the hydraulic oil introduced into the upper hydraulic chamber 14 of the gear box 10 provides a pressure which downwardly pushes the piston 12 to assist a steering force of the steering wheel. Similarly, the hydraulic oil in the lower hydraulic chamber 15 is discharged through a discharge port (not shown) in the lower hydraulic chamber 15, and returns to the reservoir tank 20 along the hydraulic oil return line 17 at the same time.
A steering angle of the steering wheel increases in a situation such as cornering, parking, and U-turn. As the steering angle increases, a desired hydraulic pressure supplied into the gear box cannot help increasing exponentially as shown in a hydraulic pressure graph of FIG. 3. In order to generate the desired hydraulic pressure, a large amount of currents flow to the electro-hydraulic power pump to generate a torque corresponding to the desired hydraulic pressure. However, since the electro-hydraulic power pump uses a power source having a voltage value of 24 V which is a battery voltage of a commercial vehicle, the following problems may occur.
First, the hydraulic pressure in the gear box should be consistently generated to maintain a steering angle of the steering wheel as the steering angle increases. However, since a large amount of current flows toward the electro-hydraulic power pump, heat is generated in wiring and a power module of the controller as well as a motor of the power pump.
Since the motor in the electro-hydraulic power pump is designed to maintain the maximum torque mode for about 10 seconds, if heat corresponding to not less than a particular level is generated as described above, the power restriction mode for the motor and the controller protection logic are executed to protect the controller and the like. As a result, the power restriction mode and the protection logic restrain the steering performance, thereby causing the steering instability.
In addition, the hydraulic pressure supplied into the gear box of the power steering apparatus is released due to the power derated by the protection logic so that the steering wheel may be rapidly released to a neutral position, thereby causing injuries to a driver.
Finally, since the electro-hydraulic power pump uses a 24 V battery power source, the battery power source is unreliable if it is over utilized.